The present invention relates to an inkjet printer and in particular to an inkjet printer for performing image recording using high viscosity ink.
Inkjet printers are generally known as a means to perform printing on various recording media. Inkjet printer refers to a printer in which image recording on a recording medium is performed when ink in the form of small droplets is emitted from the nozzle of the recording head, using a piezoelectric element or a heater for example onto a recording medium such as paper or the like, and the recording head is moved over the recording medium while the ink penetrates or is fixed on the recording medium. Inkjet printers have the advantage that printing on demand is possible since a plate-making process is unnecessary. In particular, ink jet printers using photo-curing type ink which is cured by light such as ultraviolet light have become known in recent years, and with this type of inkjet printer, light is irradiated on the ink which has been deposited on the recording medium to thereby be cured and fixed on the recording medium. As a result, it is possible for printing to be easily performed on recording media which do not absorb ink such as transparent or semi-transparent resin films and the like.
In such an inkjet printer, when high viscosity ink is employed for image recording, pressure loss is caused in the ink supplying pipe because of frictional resistance. As a result, in the case of sequential jetting of ink from the recording head nozzle, ink cannot be led to the nozzle only by the jetting force and a jetting defect may be caused. As means to prevent the jetting defect of ink caused by the pressure loss, means to reduce influence of pressure loss by providing a sub-tank for supplying ink to recording head to shorten the distance between the sub-tank and recording head and further providing a back pressure controlling pump to control the back pressure is known, as well as means to reduce frictional resistance in the ink supplying pipe by employing a larger diameter or a shorter length for the ink supplying pipe (for example, Patent Document 1).
When the ink to be used is high viscosity one, it is preferable that the diameter of the ink supplying pipe is 7 mm or larger and for example the length of it is 1 meter or shorter. However, there is a problem that it is difficult to realize this situation regarding the diameter or length of the ink supply pipe by reason of the designing difficulty of inkjet printer.
In the case of installation of a sub-tank by the recording head or a back pressure controlling pump, the structure of the inkjet printer becomes complicated. Especially, to position sub-tanks near the recording head, sub-tanks the number of which is corresponding to that of recording head needed to be mounted on the carriage and it causes upsizing and weight increase of carriage and further causes upsizing and weight increase of the apparatus itself. Furthermore, reinforcement of the driving system to drive such a large carriage is needed and there is a problem that it increases the apparatus cost.
Further, in this type of inkjet printer, image recording generally is performed while supplying ink as needed, to the recording head from the tank for storing the ink, but the quantity of ink that is stored in the ink tank decreases as image recording operations are repeated. Thus, in order to perform the image recording operation smoothly and continuously, it is necessary to accurately determine the quantity of ink remaining in the ink tank, and to replenish the ink when there is no remaining ink.
As a result, a method was carried out in which a weight sensor provided in the ink tank, detects the quantity of remaining ink by determining the change in weight of the ink tank. Alternatively a method was performed in which the quantity of ink remaining in the ink tank was calculated by keeping count of the quantity of ink which is jetted during image recording, and subtracting the counted ink quantity from the total ink quantity in the ink tank (for example Patent Document 2).    [Patent Document 1] Patent No. 2934016    [Patent Document 2] Tokkaihei No. 9-248917
However, the ink tank has various parts such an outer ink casing, joint members and the like. As a result, in the method providing a weight sensor in the ink tank, when an attempt is made to detect the weight of the ink inside the ink tank, the weight of these parts is also measured along with the weight of the ink in the tank, and this caused a problem in that weight measurement could not be performed with high accuracy. Further, there were also sometimes jetting abnormalities due to missing nozzles and the like, or ink would be forcibly absorbed from the nozzle at the time of cleaning the recording head, or sometimes there would be ink outflow as a result of air jetting. However, the method of counting the quantity of ink ejected during image recording does not count this type of ink outflow, and in addition, because high accuracy management of the ink outflow due to jetting abnormalities and ink outflow at the time of cleaning is difficult, there was a problem in that the quantity of ink remaining could not be accurately determined.